This invention relates generally to marine propulsion and more particularly to a variable pitch boat prop.
The effective angle of attack of a fixed-pitch prop changes as a function of boat speed. A fixed pitch prop is generally a monolithic body comprising a hub, affixed to a driveshaft powered by the motor, and two or more blades arranged around the hub at equal intervals. One has to choose, in selecting prop pitch, between low speed acceleration and top speed. Higher pitch props (up to a limit) produce higher top speeds from a given motor, but poorer acceleration. One generally has to compromise, selecting a prop pitch for best all-round performance: this is a somewhat subjective or empirical exercise. For a given boat, the best pitch may vary from site to site, so some boaters keep an assortment of props, to be ready for various situations.
It has long been recognized that making prop pitch variable can enable one, at least in theory, to optimize pitch for varying conditions. In practice, the pitch may be adjustable in the range of about 15.degree. to 30.degree.. Prop "pitch" is defined as the angle that a chord of the prop makes with a plane perpendicular to the axis of the prop. The term has an entirely different meaning when used in describing gear teeth.
In discussing prop geometry, it is most convenient to use a cylindrical coordinate system. The direction of the drive shaft is "longitudinal", any perpendicular to the shaft axis has a "radial" direction, and the direction perpendicular to radial, and in a plane perpendicular to the prop axis, is "tangential". For helical threads and the like, the "helix angle" is defined as the angle a tangent to the thread makes with respect to an intersecting radial plane. Prop "pitch", as used herein, means the helix angle of a chord of the prop, a chord being a line extending from the leading edge of the prop to the trailing edge.
Some prior variable pitch props had mechanical means, such as cams, for changing the blade angle. Others used resilient biasing of some type, and some had centrifugal actuation.